DAMASCUS, Syria (CNN) — A U.N. official says there are strong suspicions that Syrian rebel forces have used the deadly nerve agent sarin gas in the country’s civil war.

Carla Del Ponte told an Italian-Swiss TV station that the findings come after interviews with doctors and Syrian victims now in neighboring countries.

Del Ponte, the commissioner of the U.N. Independent International Commission of Inquiry for Syria, said the notion isn’t surprising, given the infiltration of foreign fighters into the Syrian opposition.

But rebel Free Syrian Army spokesman Louay Almokdad said rebels don’t even have unconventional weapons, nor do they want any.

“In any case, we don’t have the mechanism to launch these kinds of weapons, which would need missiles that can carry chemical warheads, and we in the FSA do not possess these kind of capabilities,” Almokdad said.

“More importantly, we do not aspire to have (chemical weapons) because we view our battle with the regime as a battle for the establishment of a free democratic state. … We want to build a free democratic state that recognizes and abides by all international accords and agreements — and chemical and biological warfare is something forbidden legally and internationally.”

The claim of rebels using sarin gas comes after months of suspicions that the Syrian regime has used the same nerve agent against rebels.

In April, the head of Israeli military’s intelligence research said the Syrian government is using chemical weapons against rebel forces.

“In all likelihood they used sarin gas,” Brig. Gen. Itai Brun said.

The Free Syrian Army’s chief of staff has also said the Syrian regime has used sarin in cities such as Homs, Aleppo and Otaiba, outside Damascus.

“We took some samples of the soil and of blood. The injured people were observed by doctors, and the samples were tested, and it was very clear that the regime used chemical weapons,” Gen. Salim Idriss told CNN’s Christiane Amanpour last month.

Sarin gas can be hard to detect because is colorless, odorless and tasteless. But it can cause severe injuries to those exposed to it, including blurred vision, convulsions, paralysis and death.

Heightened tensions between Syria, Iran and Israel

Aside from conflicting reports over who’s using chemical weapons, tensions are mounting between Syria, Iran and Israel.

Iran has no doubt Syria will deal a “crushing response” to Israel, and the Shiite regime stands ready to train its ally’s soldiers, officials in Tehran said.

The remarks, delivered by various Iranian officials and reported in state media, came Sunday after Syria claimed that Israel launched a fresh attack inside its borders.

“As a Muslim and friendly country, we stand by Syria and if there is need for training, we will provide them with necessary training,” Brig. Gen. Ahmad-Reza Pourdastan, commander of the Iranian Army’s Ground Forces, told reporters Sunday.

Around the same time, Iranian foreign ministry spokesman Ramin Mehmanparast said he had no doubt Syria and its allies will “give a crushing response to the aggressions of the Zionists,” the state-run IRNA news agency reported.

Israel has not confirmed or denied that it fired rockets that hit a military research center in the Damascus suburbs Sunday.

But Syria says it was the second Israeli airstrike in three days, and Deputy Foreign Minister Faisal al Mekdad told CNN the attack was a declaration of war by Israel.

The airstrikes have riled Syria’s allies in the region: Iran and the Lebanese militant group Hezbollah.

Why Syria matters

The Syrian civil war has pitted rebel fighters against the regime of President Bashar al-Assad, whose family has ruled the country for four decades. More than 70,000 people, mostly civilians, have been killed in the bitter conflict during the past two years.

Syria matters to Iran because it is believed to be the main conduit to the Shiite militia Hezbollah in Lebanon, the proxy through which Iran can threaten Israel with an arsenal of short-range missiles.

In 2009, the top U.S. diplomat in Damascus disclosed that Syria had begun delivery of ballistic missiles to Hezbollah, according to official cables leaked to and published by WikiLeaks.

The last thing Iran wants is a Sunni-dominated Syria — especially as the Syrian rebels’ main supporters are Iran’s Persian Gulf rivals: Qatar and Saudi Arabia.

Hezbollah’s feared scenario is Israel on one side and a hostile Sunni-led Syria on the other.

‘We are watching everything’

Sunday’s reported attack comes days after U.S. officials told CNN that they believed Israel had conducted an airstrike against Syria on Thursday or Friday.

The Israeli military did not comment on the U.S. claim either.

But Israel has long said it would target any transfer of weapons to Hezbollah or other terrorist groups.

“We are watching everything when it comes to the movement of these types of weapons. We have the means to do that,” a senior Israeli defense official told CNN’s Sara Sidner on Sunday. The official is not authorized to speak to the media.

Shaul Mofaz, a lawmaker in Israel’s Knesset, told Israeli Army Radio on Sunday that Israel isn’t meddling with Syria’s civil war. But Israel must protect itself from Lebanese militants, he said.

“For Israel, it is very important that the front group for Iran, which is in Lebanon, needs to be stopped,” Mofaz said.

Hezbollah did not immediately comment after Sunday’s claims.

‘Everything kept exploding’

Syria said Sunday’s attack targeted the Jamraya research center in the Damascus suburbs. But state media reports did not provide details about what type of research occurs in the facility, or how much damage occurred.

“Until now, the details are not clear on what happened,” al Mekdad told CNN. “Did they fire missiles? … It is not clear for me, because I don’t know how it happened, and of course it is worrying, but Israel will suffer the same.”

The blasts rocked a large military area in the suburbs of Syria’s capital, prompting terrified residents nearby to run for cover.

“Everything kept exploding over and over again,” said Anna Deeb, whose family lives just over a mile away. “We could hear gunshots, we could hear people screaming. … We didn’t know what to do, and there was a problem with us breathing because the smoke was too much.”

In January, Syrian authorities said Israeli warplanes killed two workers and injured five others when they struck the same research facility that officials said was targeted on Sunday.

A U.S. official told CNN at the time that the Syrian claims were false.